This invention relates to color imaging processors and, in particular, to photoreceptor charge control that obviates the adverse effects of photoreceptor variation inherent in the photoreceptor as the result of the manufacturing process.
Many xerographic copiers and printers maintain the charge level on the photoreceptor via feedback control by sampling the resultant charge using an electrostatic voltmeter. These InterDocument or InterPage Zone (IDZ or IPZ) readings are taken around the photoreceptor. Many photoreceptors are known to have a once-around variation in the charge level, due primarily to dielectric thickness variations commonly referred to as run-out.
Charge control was first performed in the Xerox 1075.TM. by examining the density of lightly developed images with a reflective infrared densitometer. These images were sensitive to both development field and toner concentration (the latter being controlled by examining a higher density patch). In subsequent Xerox.TM. machines (1065.TM., 5090.TM., 5100.TM., 4890.TM., 5775.TM.) one or more compact Electrostatic Voltmeters (ESV) were used to directly sense the charge levels on the photoreceptor. In each of these machines, images or test patches are placed on the photoreceptor in small regions between customer's prints, such regions being commonly known as IDZs or IPZs. The charge level of such an image is read by the ESV. These readings, sometimes filtered, are compared to a pre-established charge target and adjustments are made to the charging system to bring the readings to target. Because these readings are taken at various points around the photoreceptor, any circumferential variation in the photoreceptor charge level can affect the readings. A typical source of variation is dielectric thickness changes, established during the photoreceptor manufacturing due to run-out in the coating rolls employed to fabricate the photoreceptor. In some photoreceptors this noise can exceed an unacceptable peak-to-peak amplitude of 30 volts.
Some photoreceptors are known to possess a repeatable once-around profile but the amplitude is only about 5-10 volts. This level is at a "just noticeable difference" in color error (delta E.sub.cmc) and correction of this once-around profile is not necessary. Nor is it practicable due to the broad expanse of the charging zone that is much larger than the structure of the voltage variations.
However, it is still desirable to characterize and correct the charge readings for this variation. This will prevent the charge level from riding this profile as the charge level is maintained, thereby minimizing overall variation in the photoreceptor charge level. Uncorrected voltages will follow the once-around voltage profile of the photoreceptor and cause the average charge level to change. Corrected voltages that cause the average charge level of the photoreceptor to remain constant are quite desirable.